The spread of the Roman Empire through Europe could help explain why those living in its former colonies are more vulnerable to HIV.

The claim, by French researchers, is that people once ruled by Rome are less likely to have a gene variant which protects against HIV.

This includes England, France, Greece and Spain, New Scientist reports.

Others argue the difference is linked to a far larger event, such as the spread of bubonic plague or smallpox.

The idea that something carried by the occupying Romans could have a widespread influence on the genes of modern Europeans comes from researchers at the University of Provence.

They say that the frequency of the variant corresponds closely with the shifting boundaries of the thousand-year empire.

In countries inside the borders of the empire for longer periods, such as Spain, Italy and Greece, the frequency of the CCR5-delta32 gene, which offers some protection against HIV, is between 0% and 6%.

Countries at the fringe of the empire, such as Germany, and modern England, the rate is between 8% and 11.8%, while in countries never conquered by Rome, the rate is greater than this.

However, the researchers do not believe that the genetic difference is due to Roman soldiers or officials breeding within the local population - history suggests this was not particularly widespread, and that invading and occupying armies could have been drawn not just from Italy but from other parts of the empire.

Instead, they say that the Romans may have introduced an unknown disease to which people with the CCR5-Delta32 variant were particularly susceptible.

Disease protection

However, some researchers believe that infections may have played a role - but in reverse -increasing rather than decreasing the frequency of the variant.

Researchers at the University of Liverpool suggested that the variant may have offered protection against pandemics such as the Black Death which swept Europe on a regular basis during and after the Roman era.

These, said the Liverpool researchers, were illnesses which may have been lethal to people without the gene variant, raising its frequency from one in 20,000 people to approximately 10% in Northern Europe.

Dr Susan Scott, one of the researchers, said that the idea of Roman occupation being the driving force behind this was another theory to be considered.

"We just don't know. This is just another piece of the jigsaw, but we're waiting for the big piece of evidence which will solve this."